Modem alarm systems are known for high reliability. As a result, such systems, in particular burglar alarm systems, are often subjected to sabotage attempts which range from covering the alarm to ripping the alarm off the wall. Covering or masking of the alarm, which is a method of attacking alarm sensors such as passive infrared alarms, include covering the alarm with a box, hat or other suitable screen or by spraying the entry window of the alarm for the infrared radiation with an infrared-opaque medium. As a countermeasure for this attack mode, an additional detection system is generally incorporated into the detector, such as those which are described, for example, in EP-A-0 186 226, EP-A-0 189 536, EP-A-0 476 397, EP-A-0 499 177 or EP-A-0 772 171 (U.S. Pat. No. 5,942,976).
Another type of sabotage consists of disabling the burglar alarm by unauthorized opening of the cover. A cover contact which is opened upon removal of the cover is often used to detect such sabotage. It is also possible to couple the cover contact to the locking device between base and cover so that operation of the locking device itself is detected (see, in this regard, EP-A-0 616 307).
Yet another form of attack on an alarm system includes physically removing the alarm sensor from the wall on which it is mounted. A system for detecting this type if attached is implemented in at least one known burglar alarm system (type IR160 of Alarmcom AG, CH-8604 Volketswil). This particular detection scheme uses a panel, which is penetrated by a fastening screw of the alarm and is relatively easy to break off the base, and a contact switch. The switch is fixed on a part of the base which is not intended to be broken off and has an actuator portion that presses against a projection of the panel which is intended to be broken off. In the event of the alarm being ripped off the wall, the panel which may be broken off the base remains on the wall and the contact switch is removed from engagement with the panel, thereby opening the electrical contact of the switch. Since the alarm components, particularly passive infrared sensors, are generally intended both for flat wall mounting and for corner mounting, at least two break-off panels must be provided with associated contact switches mounted adjacent to the panels.
Although this known solution has been successful, it requires extra assembly effort and entails additional cost as a result of requiring two contact switches. Accordingly, an improved arrangement for detecting an attack on the mounting of alarm components is required.